Platelets seem to play a significant role in the development and progression of coronary atherosclerosis (CAD) and in the thrombotic and microembolic complications. Shortened platelet survival occurs in 50-75% of patients with CAD. This increased platelet consumption is partially or completely corrected with platelet inhibitor drugs. Thus, a platelet inhibitor drug trial in CAD progression is mandatory. Patients who have a high risk of progression or thromboembolic and microembolic complications may be identifiable by measurement of platelet survival time; this also has to be tested. The plasma platelet factor 4 level may correlate inversely with platelet survival and would be a faster and more economical means of identifying patients at high risk of CAD progression and complications. A prospective, double-blind platelet inhibitor drug trial with dipyridamole and aspirin versus placebo (lactose) will be conducted using selective but random allocation to treatment and control groups of patients undergoing clinically indicated left ventricular and coronary angiography because of CAD and who are subsequently treated medically. Repeat left ventricular and coronary angiograms will be performed after five years. The goal of this study is to determine the effectiveness of dipyridamole-aspirin in reducing the progression of CAD. The end points to be compared in the treatment and control groups after a follow-up of five years are coronary arteriographic evidence of progression of CAD and development of new CAD. Platelet survival with 51 chromium labeled autologous platelets and plasma platelet factor 4 levels will be determined in all patients before and after five years of treatment. We are daily identifying CAD patients from our Cardiac catheterization laboratory who are eligible for the 'Platelet Inhibitor Drug Trial in Coronary Surgery' (HL 21533). Those CAD patients who are treated without surgery would be candidates for this study, and those two drug trials could be run in parallel.